I suggest that Austin vegetarians bookmark this neat web site:
"Veg Out! Austin... where herbivores eat out in Central Texas"
Besides mentioning Magnolia Mud, a recent article by Ben recommends 5 tops picks for vegetarian nachos. You can read all about his top 5 list here.
Just briefly, here are the recommended choices, in no particular order:
Alamo Drafthouse at the Ritz, Nacho Libre with grilled tofu
Freebird's World Burrito, Nachos with black beans and queso
Magnolia Cafe, Super Nachos
ZuZu's, Black Bean Nachos
Chuy's, Special Nachos
Good to know!